1. Field of the Invention
This invention relates to a semiconductor device and its manufacturing method, specifically to a high withstand voltage MOS transistor and its manufacturing method.
2. Description of the Related Art
The high withstand voltage MOS transistor has a high source-drain withstand voltage (BVDS) or a high gate withstand voltage, and is used in various kinds of drivers such as an LCD driver and an EL driver, power supply circuits and the like.
FIG. 6 is a cross-sectional view showing a structure of an N-channel type high withstand voltage MOS transistor according to a conventional art. A gate insulation film 101 and a thick field insulation film 102 are formed on a surface of a P-type semiconductor substrate 100. A gate electrode 103 is formed on the gate insulation film 101 and on an adjacent portion of the field insulation film 102. In the surface of the semiconductor substrate 100, there are formed a high impurity concentration (N++ type) source layer 104 and a low impurity concentration source layer 105 adjacent one end of the gate electrode 103.
A high impurity concentration (N++ type) drain layer 106 is formed in the surface of the semiconductor substrate 100 separated from the other end of the gate electrode 103. A low impurity concentration (N− type) drain layer 107 that is lower in impurity concentration and deeper in diffusion depth than the high impurity concentration drain layer 106 is formed in a region extending from beneath the gate electrode 103 to beneath the field insulation film 102 and the high impurity concentration drain layer 106. The high impurity concentration drain layer 106 is formed in the low impurity concentration drain layer 107. The source region and the drain region are made of a so-called LDD (Lightly Doped Drain) structure that is composed of a high impurity concentration portion and a low impurity concentration portion, as described above. A sidewall spacer film 108 made of a silicon nitride film or the like is formed on a sidewall of the gate electrode 103.
The conventional high withstand voltage MOS transistor described above obtains the high source-drain withstand voltage because a drain electric field is eased by extending a depletion layer into the low impurity concentration drain layer 107 when a high voltage is applied to the high impurity concentration drain layer 106. Also, it has a structure sturdy against breakdown of the gate insulation film 103, because the gate electrode 103 extends from the gate insulation film 101 to the adjacent portion of the field insulation film 102.
Technologies described above are disclosed in Japanese Patent Application Publication No. 2002-134738.
However, there has been a problem that the conventional transistor structure described above does not have enough withstand voltage against electrostatic discharge (hereafter referred to as ESD withstand voltage). For example, according to typical electrostatic discharge tests performed by the inventors, the ESD withstand voltage based on a human body model (HBM) is less than 200 volts and the ESD withstand voltage based on a machine model (MM) is less than 50 volts, which are not high enough. Therefore, this invention is directed to a transistor structure that improves the ESD withstand voltages.